The present invention is directed to a novel process for fabricating a nonaqueous emulsion feedstock for use in the preparation of rubber-modified thermoplastic copolymers (two or more monomer components), a process for the preparation of rubber-modified thermoplastic copolymers and the new and improved rubber-modified copolymers produced by this method. In particular, the polymerization process of the present invention is characterized by the use of a single nonaqueous stable rubber-containing emulsion feedstock comprising (1) a mixture of free radical polymerizable unsaturated organic monomers which mixture is substantially a nonsolvent for the rubber, (2) a rubber and (3) a block copolymer. The rubber-modified thermoplastic material of the present invention comprise the polymerization product of the nonaqueous emulsion feedstock. In particular the rubber-modified copolymers of the present invention may comprise the polymerization product of a monoalkenyl aromatic monomer, a C.sub.1 to C.sub.3 alkyl methacrylate and/or acrylonitrile, an unsaturated dicarboxylic anhydride, rubber and a block copolymer. A rubber-modified thermoplastic copolymer of the present invention comprising a monoalkenyl aromatic monomer, a C.sub.1 to C.sub.3 alkyl methacrylate, unsaturated dicarboxylic anhydride, rubber, and block copolymer is characterized, by improved heat distortion temperature, transparency and impact strength.
Generally, rubber-modified copolymer compositions have been prepared by various procedures such as bulk inversion polymerization, aqueous emulsion polymerization, or solution polymerization. In particular, rubber-modified terpolymers such as styrene/maleic anhydride/alkyl methacrylate have been made by solution polymerization as evidenced by U.S. Pat. No. 4,341,695.
Each of the above-mentioned polymerization procedures has severe limitations. For example, aqueous emulsion polymerization or solution polymerization although enabling suitable rubber dispersion have the obvious disadvantage that the water or solvent which is present during the processing must be removed resulting in a severe economic disadvantage.
Bulk inversion polymerization usually results in a matte finish polymer due to the large (&gt;1 micron) nonuniform particle size of the rubber formed. Furthermore, the process cannot be employed in conventional reactors where the monomer mixture is a nonsolvent for the rubbery component. Bulk polymerization which employs an extruder as the polymerization vessel overcomes some of these feedstock miscibility problems as well as alleviating some of the problems associated with aqueous emulsion or solution polymerization. In particular, reactor extruders of the twin screw design (see U.S. Pat. No. 4,463,137) provide sufficient shearing action during polymerization of the monomers that the resulting rubber-modified copolymer has smaller size rubber particles dispersed therein. However, there are some problems associated with reactor extruder polymerization. For example, the two feed solution described in U.S. Pat. No. 4,463,137 has potential pumping problems associated with the extremely high viscosity differential between the two feeds making it difficult to control the feed ratios. In addition, the dispersion of the intractable rubber rich phase in a desirable manner in the polymerized product is particularly difficult to achieve due to the rapid rates advantageously used in the reactor-extruder polymerization process. That is, there is not enough time for uniform dispersion of the rubber particles resulting in poor rubber phase dispersion in the finished products.
The present invention is directed to a procedure which alleviates the problems associated with the above-identified procedures and the novel polymerization product produced by this procedure. The present invention provides a means for preparing a single feed mixture having a desired low viscosity and predetermined rubber particle size. This single feed mixture results in an evenly dispersed rubber phase within the bulk polymerization product. The resulting rubber-modified polymerization product of the present invention possesses unexpectedly superior physical characteristics compared to the rubber-modified polymerization products known in the art.